The important biological roles that peptides and polypeptides play as hormones, enzyme inhibitors, substrates, neurotransmitters, and neuromediators has led to the widespread use of peptides or peptide mimetics in medicinal chemistry as therapeutic agents. The peptide's bioactive conformation, combining structural elements such as alpha helices, beta sheets, turns, and/or loops, is important as it allows for selective biological recognition of receptors or enzymes, thereby influencing cell-cell communication and/or controlling vital cell functions, such as metabolism, immune defense, and reproduction (Babine et al., Chem. Rev. (1997) 97:1359). The alpha-helix is one of the major structural components of peptides. However, alpha-helical peptides have a propensity for unraveling and forming random coils, which are, in most cases, biologically less active, or even inactive, and are highly susceptible to proteolytic degradation.
Many research groups have developed strategies for the design and synthesis of more robust peptides as therapeutics. For example, one strategy has been to incorporate more robust functionalities into the peptide chain while still maintaining the peptide's unique conformation and secondary structure (see, for example, Gante, J. Angew. Chem. Int. Ed. Engl. (1994) 33:1699-1720; R. M. J. Liskamp, Recl. Tray. Chim. Pays-Bas 1994, 113, 1; Giannis, T. Kolter, Angew. Chem. Int. Ed. Engl. 1993, 32, 1244; P. D. Bailey, Peptide Chemistry, Wiley, New York, 1990, p. 182; and references cited therein). Another approach has been to stabilize the peptide via covalent cross-links (see, for example, Phelan et al. 1997 J. Am. Chem. Soc. 119:455; Leuc et al. 2003 Proc. Nat'l. Acad. Sci. USA 100:11273; Bracken et al., 1994 J. Am. Chem. Soc. 116:6432; Yan et al. 2004 Bioorg. Med. Chem. 14:1403). However, the majority of the reported methodologies involve use of polar and/or labile crosslinking groups.